The invention relates to a device for the monitoring of objects and/or persons according to the preamble of Patent Claim 1.
For safeguarding of objects from unauthorized removal, so-called resonator tags are known which are attached to the object in question to be safeguarded and which contain a resonant circuit that, when the object is taken through a "gate", causes interference at an RF field generated there, which in turn triggers an alarm. If, however, a shoplifter succeeds in removing the tag from the object to be safeguarded before going through the gate, the theft cannot be detected. In addition, it is a pre-requisite for resonator tags of this kind, for each exit from the area (shop) to be monitored to have such a gate, which, due to the high costs of the gates, is economically unjustifiable, for example, for larger department stores or even factory premises.
A device of the type mentioned at the start is known from German Offenlegungsschrift No. 3,119,112, which itself outputs an (acoustic) alarm, either when the security tag is removed from the object to be safeguarded or when the tag is taken out of an area in which, by means of an RF transmitter, an RF field is generated which is detected by an RF receiver arranged in the tag and used to suppress an alarm. A technically minded shoplifter can however very easily carry a transmitter (emitting over a broadband) and remove the "safeguarded" object complete with the security tag kept silent by means of his or her transmitter. If, moreover, the known device is to be used to monitor not objects but persons,--e.g. visitors to a larger plant who should only have access to certain areas--, then this is only possible to a limited extent, since at best individualization of the security tags (permitted/forbidden areas are not the same for each visitor) is possible using matched RF circuits, which again leads to economically unjustifiable high costs for the security tags.